[GitHub] [kafka] wanwenli edited a comment on pull request #10686: KAFKA-4083: prevent changing replication factor via partition reassignment

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wanwenli edited a comment on pull request #10686:
URL: https://github.com/apache/kafka/pull/10686#issuecomment-844963635


   > While I agree that all partitions ought to have the same number of replicas, I think this fix is enforcing something different: That you can't change the RF for a partition. While that prevents the partitions getting different RFs from each other if they had the same RF originally it also prevents changing the RF for all the partitions of a topic, which we should support.
   > 
   > There's also the use case where you want to change the RF for a topic, but because it has a large number of partitions you want to do it in batches, using multiple reassignments. I think Cruise Control does this kind of batching, for example. Supporting that requires that, for a time, the topic has partitions with different numbers of replicas.
   
   Thanks, @tombentley. I totally agree that changing RF via partition-reassignment API is a very valid use case. May I add this piece of logic as a solution:
   
   If a partition reassignment attempts to change RF, its plan must change the RF of all partitions of a topic to the _same_ number. What do you think?
   
   One more thing to confirm with you: `partitionAssignments` in KafkaController contains the all topic partitions in the entire cluster, right? I find in [this line](https://github.com/apache/kafka/blob/7f91d2935f821c55afce520debb7bc97d60185d4/core/src/main/scala/kafka/controller/KafkaController.scala#L906) that all topic partitions are loaded into the controller upon initiation.


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